Feature: control python version upon install, set max version to gym

[royerk]

What

Given the pickiness of the entire setup in order to be consistent with kaggle's env:

• check python version upon running python setup.py install
• install a version of gym < 0.20.0 to enforce compatibility with SB3

This should save some trouble to new users or when the repo is updated & a re-install is required.

Test

• [x] install from scratch in an empty virtualenv, gym version is ok
• [x] install with an invalid python version to trigger error
• [x] run train.py successfully

[royerk]

I have made a mess in my git. Allow me to close this one and open a clean PR without conflicts. Really sorry about that.